Introduction to Sound Synthesis Mixing and dynamics
Compression
A compressor turns a signal down when it gets louder than a threshold, by an amount set by the ratio. Attack and release set how quickly it reacts. The difference between the loud and quiet parts becomes smaller, and once the signal is turned back up the quiet parts are louder than before, so it sounds more even.
A compressor contains an envelope follower that measures the signal's level, and a VCA or other gain stage that sets its gain. While the measured level is below the threshold, the gain stays at unity. Above it, the gain stage turns the signal down so that the level rises by only a fraction of what it would otherwise: at a ratio of 4:1, every 4 dB the input rises above the threshold becomes 1 dB at the output. Compression reduces dynamic range, the difference between the loudest and quietest moments, and makeup gain afterwards brings the whole signal back up, so that the quiet parts end up louder than before.
The tool below compresses a drum loop or plucks. The left plot is the compressor's transfer curve: output level against input level. The right plot shows how much the compressor is turning the signal down, moment by moment.
A compressor
Drums or plucks pass through a compressor with threshold, ratio, knee, attack, release and makeup gain. The transfer curve and the live gain reduction are drawn, and the compressor can be bypassed to compare.
Listen for With a low threshold and a high ratio, the peaks of the kick and snare are turned down, and with makeup gain the quieter hi-hats come up relative to them. A slow attack, around 30 ms, lets the start of each hit through before the gain falls, so the drums keep their sharp onsets. With a fast release the level rises audibly between hits, which is heard as pumping. With a slow release the gain stays lowered across several hits and the level changes less. Toggle Bypass to compare.
Threshold and ratio
The threshold sets where compression begins, and the ratio sets how strong it is. On the transfer curve, input levels below the threshold follow a line parallel to the diagonal, where output equals input, raised by the makeup gain. Above the threshold the curve bends to a shallower slope: at 2:1 it rises half as fast, at 10:1 a tenth as fast. A ratio of 10:1 or more keeps the output close to the threshold (at 10:1, an input 20 dB over the threshold comes out 2 dB over), and compression at such high ratios is called limiting. The knee sets how sharply the bend happens. A soft knee spreads it over a range of levels around the threshold, which makes the start of compression harder to hear than with a hard knee.
Attack and release
A drum hit is loudest in its first few milliseconds, so the attack time sets whether that first moment is turned down. With an attack of 20 or 30 milliseconds, the first moment passes before the gain falls, so the hit keeps its sharp onset and the rest of the hit is compressed. With an attack of a millisecond or less, the peak itself is turned down and the hit becomes flatter. Release sets how quickly the gain recovers. With a fast release the level rises quickly between hits, which is heard as pumping. With a slow release the gain stays lowered across several hits, so the level varies less, but quieter sounds that follow a loud one are turned down as well. SampleStack's interactive guide to compression covers the same controls as they apply to preparing samples.
Sidechains
Many compressors can measure one signal and turn down another, and the input for the measured signal is called the sidechain. A pad or a bass line compressed by a sidechain from the kick drum drops on every kick, the ducking described in envelope followers. A sidechain through an equaliser that boosts the frequencies of the "s" and "sh" sounds in a vocal makes a de-esser, which turns down only the moments when those frequencies are strong. Compression on individual sounds, and then gently on the whole mix, is a central part of mixing and mastering.
Common questions
What does a compressor do?
It turns a signal down automatically when it goes above a set level, the threshold. The ratio sets how much: at 4:1, a signal that goes 8 dB over the threshold comes out only 2 dB over. The result is a smaller difference between the loud and quiet parts, which is then usually turned up as a whole.
What do attack and release mean on a compressor?
Attack is how quickly the compressor turns the signal down once it goes over the threshold, and release is how quickly it lets the level back up once the signal falls below. A slow attack lets the start of each drum hit through at full level before the gain falls, so the hit keeps its sharp onset. A fast attack turns the onset down as well, and the hit sounds flatter.
What is the knee on a compressor?
It is the width of the region around the threshold where compression begins. With a hard knee, compression starts abruptly at the threshold. With a soft knee, it begins gradually below the threshold and reaches the full ratio above it, which makes the start of compression harder to hear.
What is makeup gain?
It is gain added after the compressor to bring the overall level back up, since compression lowers the loud parts. With makeup gain, the quiet parts end up louder than before and the loud parts about the same, so the average level rises relative to the peaks and compressed sounds seem louder and more even.
Published Glossary